There is known an art of stretching a belt from a shoulder of a seat back to a front end of a seat cushion upon prediction of a lateral collision, so as to prevent a seated passenger from moving laterally of a vehicle (e.g., see Japanese Patent Application Publication No. 2007-8194 (JP-2007-8194 A)). Besides, there is known an art of preventing a side airbag from moving outward in a vehicle width direction with the aid of a belt that is stretched from an upper end of a seat back to a front end of a seat cushion in the event of a lateral collision (e.g., see Japanese Patent Application Publication No. 2009-29182 (JP-2009-29182 A)).